Biology of Disease

4.2 Types of Immune Defense

The immune system has to distinguish between the cells and macromolecules

that make up the body that is, self, and those that are foreign or nonself.

Immunological defenses are usually classified as nonspecificandspecific.

Nonspecific defenses constitute a first line of defense and are available

immediately any foreign material, including substances such as wood splinters

as well as microorganisms, enters the body. Nonspecific defenses include

responses such as inflammation, a rapid immediate response to tissue damage,

and the acute phase response, a relatively rapid response to infection. In

specific immunological defense, cells of the immune system recognize not just

individual microorganisms, but also the particular proteins or glycoproteins

found on that microorganism. This type of defense may take several days to

become effective, depending on whether the immune system was previously

exposed to that specific microorganism but, once activated, results in a long-

lasting immunity to it. This immunity can be humoral, in that it involves the

production of antibodies and/or cell-mediated, which involves the production

of cells that kill or recruit other cells to kill the infected cells. Humoral immunity

is effective against microorganisms that do not invade cells and this includes

most bacteria and multicellular parasites (Chapter 2). Cell-mediated immunity

is effective against intracellular parasites, including viruses and some bacteria.

However, these two types of specific immunity are not mutually exclusive and

usually both types are activated on exposure to the infectious agent.

4.3 Nonspecific Defenses

Microorganisms are prevented from entering the body by structural barriers,

such as the skin and the mucosal membranes. These barriers are further

protected by chemical secretions, for example lactic acid in sebum and

hydrochloric acid secreted in the stomach. Furthermore, mucus, secreted by the

mucosal membranes that line the urogenital, respiratory and gastrointestinal

tracts, contains lysozyme, an antibacterial enzyme that catalyzes the hydrolysis

of the peptidoglycan wall of some types of bacteria. Should microorganisms

breach these barriers, other antimicrobial proteins such as complement and

interferons may be activated and/or produced. Finally, there are a number of

cells that have a role in the elimination of microorganisms. These cells may

be found in the blood, although a number are also found in the solid tissues.

Some blood cells move between the blood and the extravascular tissues,

particularly during an infection.

Interferons

Interferons (IFNs) are families of inducible, secretory proteins produced by

eukaryotic cells in response to viral infections and other stimuli. They disrupt

viral replication in neighboring healthy cells by inducing those cells to produce

enzymes that inhibit replication of viral nucleic acid and the production of

viral proteins. There are three major families of interferons: IFNs @AandF.

Interferons @ and Aare produced by cells that have been infected with a virus.

They are the predominant forms produced by leukocytes and fibroblasts

respectively, although both types are produced by other viral infected cells

in the body. Interferon F is produced by cells of the specific immune system

in response to any agent, whether bacterium, virus or foreign protein, which

stimulates that system.

Interferons belong to a large family of proteins called cytokines, a general name

given to proteins secreted by cells that stimulate activities in other cells after

binding to receptors on their surfaces. Cytokines act at low concentrations and

may stimulate different activities depending on the type of target cell. Many

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